Pope Leo III, who had been Pope since 795, became embroiled in controversy during his tenure in Rome. Following allegations against him, particularly concerning his moral character and accusations of perjury, he faced mob violence from citizens who disapproved of his leadership. This culminated in a physical assault during which he sustained significant injuries, including facial disfigurement. Fearing for his life and authority, he decided to seek refuge with King Charlemagne, who was at that time consolidating his power over much of Western Europe.

Upon reaching Paderborn, Leo III was received by Charlemagne, who was keen to bolster his own legitimacy through a strong alliance with the papacy. This interaction set the stage for a mutual relationship that would significantly influence the subsequent history of Europe. Charlemagne recognized the Pope's plight and agreed to protect him. This decision would eventually lead to the Pope reinstating Charlemagne's rule over the Frankish Empire, solidifying the church's influence in secular matters and the emerging concept of Christendom.

Leo's stay at Charlemagne's court lasted several months, during which he regained his strength and re-established his authority. His return to Rome was marked by a formal ceremony, endorsed by Charlemagne, thereby reasserting the Pope's power. This moment solidified the intertwining of papal and royal authority, establishing a precedent for future relationships between the church and European monarchs, particularly within the Holy Roman Empire.